Atomic interferometry is a known technique with applications in gyroscopes and accelerometers, as well as in other instruments for sensing and metrology. According to known principles of light-pulse atomic interferometry (LPAI), selected atoms are placed in a coherent superposition of different internal states, which oscillate as each atom simultaneously traverses two possible paths between common initial and final points in phase space. At the final point, the two paths interfere. Phase differences between the two paths induced by acceleration or rotation are manifested as shifts in the relative populations of the different internal atomic states, as revealed by a state-selective detector such as a fluorescence or laser-absorption detector.
More specifically, a stimulated Raman transition entangles momentum states (which are external) with spin states (which are internal) so that the atoms in the respective arms of the interferometer are in different momentum-spin states. The first pulse of a so-called “π/2-π-π/2” sequence of optical pulses splits the atomic phase trajectories into the two interferometric “arms” by exciting some atoms from a lower to a higher hyperfine level of the atomic ground state by means of a Raman transition in which photonic recoil induces a corresponding momentum state. (The laser that induces the Raman transition is detuned by a small increment matched to a designated Doppler shift, so as to select for a particular narrow range of initial momenta.) The second pulse exchanges the internal states relative to the external states (or vice versa) so that the two arms will converge. Depending on its phase, the third pulse places the atom in the upper or lower hyperfine state for subsequent detection. Sensing of acceleration or rotation is possible because these conditions cause different phase shifts in the different arms of the interferometer.
It should be understood that the preceding discussion is directed to atomic Raman transitions for purposes of illustration only, and not by way of limitation. Raman-active molecules that exhibit appropriate transitions may also be useful in this context, and are not excluded from the scope of the present invention. Accordingly, references to an atomic vapor or a warm atomic vapor should be understood to include molecular vapors.
As the term is used herein, any atomic or molecular species is “Raman-active” if it exhibits at least one stimulated Raman transition.
As will be seen below, an exemplary embodiment of the present invention employs rubidium vapor as the Raman-active atomic species. It should be understood, however, that any of various atomic (and possibly molecular) species may be used, provided only that they provide suitable optical transitions.
FIG. 1 provides an atomic energy-level diagram of rubidium, illustrating among other things a hyperfine splitting F=1, F=2 of the atomic ground state of rubidium. The cooling beam along with the repump beam (respective frequencies νcool and νrepump, as indicated in the figure) are used to recapture, launch and cool the atoms. The optical pumping pulse (frequency νpump, as indicated in the figure) along with the repump beam (frequency νrepump) are used to initially populate the F=2, mF=0 state. The F=1 state is then populated by the first π/2 pulse via a Raman transition stimulated by the Raman pulse (frequencies ν1 and ν2 as indicated in the figure), which counterpropagate through the vacuum cell. The probe pulse (frequency νdetect, as indicated in the figure) along with the repump pulse (frequency Vrepump) is used to detect the post-interferometer population of atoms in the F=2 and F=1 states via fluorescent emission. The frequency offset δ is the detuning of the Raman pulse to accommodate Doppler shift of the desired velocity class. The frequency offset Δ is an intentional detuning that controls the frequency of oscillation of the superposed atomic state and mitigates the effect of spontaneous emission.
Until recently, atom interferometer accelerometers have operated at data rates at or below a few Hertz, which is too slow to be useful for application such as navigation on dynamic platforms. Accordingly, there remains a need for atom interferometers that can provide data rates great enough for applications in navigation.